Solder paste and residue

ABSTRACT

This invention relates to an on-line statistical process control device for solder paste and residues. The invention consists of electronics hardware, software, and probing systems. The electrical hardware of the invention provides voltage and current measurements of solder paste materials, the software of the invention controls the hardware, provides real-time complex, non-linear least squares curve fitting for equivalent circuit analysis, data storage and retrieval of circuit parameters and behavior, and statistical process control tracking and charting. The probing systems of the invention allows for 2, 3, and 4 probe surface and bulk measurements of the solder paste and residues.

This application is a division of Ser. No. 08/874,056, filed Jun. 12,1997, now U.S. Pat. No. 6,005,399; which is a division of Ser. No.08/393,765 filed Feb. 24, 1995, now U.S. Pat. No.5,656,933.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout payment of any royalties thereon or therefor.

FIELD OF THE INVENTION

This invention relates to an on-line statistical process control devicefor solder paste and residues. The invention consists of electronicshardware, software, and probing systems. The electrical hardware of theinvention provides voltage and current measurements of solder pastematerials, the software of the invention controls the hardware, providesreal-time complex, non-linear least squares curve fitting for equivalentcircuit analysis, data storage and retrieval of circuit parameters andbehavior, and statistical process control tracking and charting. Theprobing systems of the invention allows for 2, 3, and 4 probe surfaceand bulk measurements of the solder paste and associated residues inmanufacturing.

BACKGROUND OF THE INVENTION

A need for real-time solder paste process control is critical due to thedynamic nature of solder paste. Both the rheology and the solderabilityof solder paste can change drastically during manufacturing. Thesedynamic changes are dependent on the manufacturing environment and onthe characteristics of the specific paste being used. An environmentwith high humidity can cause an increase in slump and, potentially, anincrease in the probability of solder balls due to absorbed moisture. Inaddition, solder paste, over time, can either increase or decrease inviscosity; this classifies the paste as being thixotropic or rheopectic,respectively. The dynamic change in rheology can cause significantproblems in the printability and slump of the solder paste. Lastly, anychanges in the flux material can effect the solderability of the solderpowder and can also have an impact of the rheologic nature of the pastedue to the excessive build-up of reaction products between the fluxactivators and the metal oxides (such as S_(π)O and/or S_(π)O₂).

Current methods of measuring the rheologic characteristics of solderpaste entail the use of a viscometer. A viscometer is capable ofmeasuring the viscosity of a solder paste material at a differentshearing rates. Thus, the viscosity of a solder paste can be tracked ata reference shear rate and the thixotropic character of the solder pastecan be tracked by calculating the change in viscosity over a change inshearing rate. Currently, there are two viscometers commonly used in theindustry: a Malcolm Viscometer and a Brookfield Viscometer. Both ofthese systems allow a manufacturer to quantify both viscosity andthixotropic behavior.

The Brookfield viscometer uses a T-type spindle that rotates at a givenrate in rotation and z-height while the Malcolm uses a screw-typespindle that causes the solder paste to pump up through the spindle tomake a torque/viscosity measurement. The advantage of the Brookfield isin its acceptance by the industry and its maturity in quantifyingviscosity. Due to the lack of controlled shearing with the T-typespindle, the Brookfield has limitations in measuring thixotropicbehavior. The Malcolm is a relatively new viscometer design that wascentered around the needs of solder paste rheologic measurements. TheMalcolm is well designed to handle both viscosity and thixotropicmeasurements but does not have the same acceptance as the Brookfield inthe electronics manufacturing industry. In both cases, neither system iscapable of measuring the rheologic properties of solder paste onceplaced in a manufacturing environment. These systems are principallydesigned to make bulk rheologic measurements as an incoming inspectiontool and typically require a significant amount of paste for an accuratemeasurement.

There are no other known electrical systems that measure and controlsolder paste materials currently available to the industry. Related U.S.Patents include U.S. Pat. No. 5,103,181 issued Apr. 7, 1992 to Gaisfordet al. which is a composition and monitoring process that uses impedancemeasurements. In U.S. Pat. No. 4,939,469 a method for the evaluation ofprinted circuit boards is disclosed that uses the impedance spectra ofthe board to evaluate a number of characteristics such as moisturecontent. And U.S. Pat. Nos. 3,482,161, 3,440,529, and 3,448,380 all usespectroscopic analysis for sample analysis.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail, by way of example only, withreference to the following drawings. Additional features necessary tothe invention will be evident from the drawings.

FIG. 1 is a Block Diagram of the Instrumentation System

FIG. 2 is the Solder Paste Process Control Interface Board

FIG. 3 is the Basic Menu Functionality of the Solder Process ControlSoftware

FIG. 4 is the Master Flow Chart of Impedance Spectroscopy Measurements

FIG. 5 is the Data collection flow chart for the 2-pole measurements

FIGS. 6A-B is the data collection flow chart for the 4-pole measurementswith one lock-in amp

FIGS. 7A-B is the data collection flow chart for the 4-pole measurementswith two lock-in amps

FIG. 8 is the set sensitivity flow chart

FIG. 9 is the flow chart for the capture voltage and phase

FIG. 10 is the flow chart for the non linear least squares curve fit

FIGS. 11A-B is the flow chart for the curve fit minimization(Marquardt-Levenberg method)

FIGS. 12A-B is the flow chart for the test statistical process controllimits

FIG. 13 shows the approach used to characterized solder paste materialsusing impedance spectroscopy techniques

FIG. 14 is a graph showing solder balls versus the resister in theequivalent circuit

FIGS. 15A-D shows solder balling characteristics on a scale from 1 to 4

FIG. 16 is a graph showing solder balling versus 4-probe bulk timeconstant

FIG. 17 is an equivalent electrical circuit for AIM 437 water solublesolder paste

FIG. 18 is and equivalent circuit for Multicore LG02 Water Solublesolder paste

FIG. 19 shows the change made in the Tau-2 (diffusion behavior) mappedalong side the change in the thixotropic index. (in a 2-probemeasurement)

FIG. 20 Equivalent Electrical circuit for mapping oxides in a 0%Multicore RMA Flux

FIG. 21 show the surface 4-probe

FIG. 22 shows the ¾-pole bulk probe

FIGS. 23A-E shows other views of the ¾-pole bulk probe of FIG. 22.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The Solder Paste Process Control System is functionally divided into 4subsystems: a) Instrumentation Hardware, b) Software, c) ManufacturingInterface & Modeling, and d) Probing Hardware

a) Instrumentation Hardware

The instrumentation hardware subsystem is responsible for interfacingwith the probing system switching series (current) resistors in thecircuit and voltage and phase measurements (which are, in turn,translated to real and imaginary impedance characteristics in software).There are three basic subunits in the preferred instrumentationsubsystem as shown in FIG. 1: 1) Stanford Research Systems SRS810 1, 2)the interface board 2, and 3) Personal Computer 3 (486-xx) with IEEE-4884 and digital I/O capability 5. The SRS 810 1 is computer controlled viathe IEEE-488 bus 6 and is responsible for making voltage and phasemeasurements. The interface board is responsible for switching knownresistor in series with the solder paste (for current measurements) andproviding an interface for 2, 3, and 4 pole probes to the SRS810 1. Thecomputer 3 is responsible for logic associated with controlling the timeconstant, resolution, filtering, frequency selection, voltagemeasurements, and phase measurements. The computer 3 processes thevoltage and phase information to create a real and imaginary impedancecharacteristic. In addition, the computer 3 provides the relay firinglogic that places the appropriate series resistor in the circuit forcurrent measurements (magnitude and phase) and for voltage measurementsacross the solder paste or residue material (magnitude and phase). Thecomputer 3 uses the National Instruments PC-DIO-24 digital I/O card 5 tosend the logic for controlling the relays. See FIG. 2 for an detailedschematic of the interface card 2.

The Stanford Research System SRS 810 1 is an off-the-shelf lock-inamplifier that is capable of a) generating sine and TTL signals from 10mHz to 100 kHz and b) making voltage and phase measurements in referenceto the internally generated signal or an external signal. In addition,the SRS 810 1 has the capability of implementing different timeconstants, discrete resolution, and filtering options.

The computer 3 is responsible for implementing stored logic to controlthe National Instruments PC-DIO-24 digital input/output card thatultimately controls the relay firing sequence during a test. The relayfiring sequence used to select a series resistor for a particularmeasurement in the Solder Paste and Residue Measurement System. Inaddition to selecting the series resistor, the computer 3 is responsiblefor switching between the series resistor and the solder paste/residuesample in order to measure both voltage and current through the solderpaste or residue. From these voltage and current readings, the real andimaginary impedance of the sample can be calculated. The calculationsused are:$I_{S} = {\frac{V_{R}/\underset{\_}{\Phi_{R}}}{R_{S}/\underset{\_}{0{^\circ}}} = {{\frac{V_{R}}{R_{S}}/\underset{\_}{\Phi_{R} - {0{^\circ}}}} = {\frac{V_{R}}{R_{S}}/\underset{\_}{\Phi_{R}}}}}$

$Z_{Sample} = {\frac{\quad {V_{Z}/\underset{\_}{\Phi_{Z}}}\quad}{{V_{R}/R_{S}}/\underset{\_}{\Phi_{R}}} = {{R_{S}\left( \frac{V_{Z}}{V_{R}} \right)}/\underset{\_}{\Phi_{Z} - \Phi_{R}}}}$

The computer 3 also is responsible for system calibration, datamanagement, data analysis, data representation (i.e. graphing, etc.),and solder paste control calculations.

The interface board's 2 primary role is do a discrete switch of theinput to the SRS 810 1, to place high resolution discrete resistors inthe measurement circuit for current measurements, and to interface withthe probing system 7 for measuring solder paste and residues. There arefive main functional blocks of the interface card's 2: (1) the computerinterface 20, (2) inputs and outputs from the Stanford Research modelSR-810 lock-in amp (these inputs come into the interface board at A 28,29 and B 30, 31, (3) the programmable load cell 40, (4) the relayswitching and driver circuitry 45, and (5) the power supply 47.

Computer Interface

The computer interface 20 consists of a digital I/O card 21 similar tothe National Instruments PC-DI/O-24 resident in the computer andconnected via a 50 conductor cable 22 to the interface board. Theinterface cable 22 mates via an interlocking connector mounted to theback side of the board. The 50 lines are configured as 24 channels withseparate grounds and a 5 volt output with ground. All the ground linesfrom this connector are tied to the system ground. The 24 sigle channeloutputs are TTL logic compatible and buffered through inverters whichactuate drivers for the relays which facilitate input mode, and loadcell switching.

Instrument & Probe Interface

The probe interface 23 consists of four panel mounted test points(TP1-TP4) used to connect the probe and sample under test to themeasurement system. TP1 24 is the Working electrode, TP2 25 is theWorking Sense electrode, TP3 26 is the Reference electrode, and TP4 27is the Counter electrode. The interface to the SR810 lock-in amplifierconsists of 6 jacks: J1 28 connects to the A channel on amp 1, J2 29 toB channel on amp 1, J3 30 to A channel on amp 2, J4 31 to B channel onamp 2, J5 32 to Sine Out on amp 1, and J6 33 to Ref In on amp 2.

If two lock in amplifiers are used in the system (such as a lock in ampwith a different frequency range), relays in the input switching relays35 are used to select which lock-in amplifier is switched “in circuit”to perform the measurement. An additional connection is made from thesine out 32 of lock in amplifier one (master) to the Ref In 33 input onlock-in amplifier two (slave). The two inputs “A” 28 and “B” 29 form thedifferential input to the lock-in amp. The sine out 32 from the lock-inamp is used as the stimulus to the sample under test via TP1 24.

Relays in the input switching relays 35 are configured to connect input“A” 28 or 30 and “B ” 29 or 31 to TP2 25 and TP3 26 in the de-energizedposition to facilitate the 4 pole voltage measurement. Relays in theinput switching relays 35 are also configured to switch the “A” 28 or 30input to TP3 26 and the “B” 29 or 31 input to LO side of the generatorfor the 4 pole current measurement.

Relays in the input switching relays 35 are also configured to switchthe “A” 28 or 30 input to TP1 24, and the “B” 29 or 31 input to TP4 27to configure the system for a 2 pole voltage measurement (Thismeasurement is only made when the solder paste impedance is <500 KW).The 2 pole current measurement is facilitated by energizing the inputswitching relays 35 to connect the “A” input 28, 30 to TP4 27 and the“B” 29 or 31 input to the LO side of the generator.

Relay in the input switching relays 35 switch the “A” input 28 or 30 toTP3 26 and the “B” input 29 or 31 to TP4 27 to facilitate a 3 polevoltage measurement. This places the analyzer inputs across the probeinterface. By energizing the input switching relays 35 to connect the“A” input 28 or 30 to TP4 27 and the “B” input 29 or 31 to the LO sideof the generator facilitates the 3 pole current measurement.

In the test mode, only one of the five input switching relays will beactivated at any one time. Relays in the input switching relays 35 aretied in a parallel configuration and are actuated as one 4-poledouble-throw relay. These set of relays switch the measurement fromlock-in amplifier number one (Master) to lock in amplifier number two(Slave). A relay in the input switching relays 35 will switch themeasurement system to a 3 pole voltage configuration. And a relay mustbe de-energized prior to changing the input switching mode. A relay willalso switch the system to a 2 pole voltage configuration, and must be deactivated prior to changing input switching modes. Relays in the inputswitching relays 35 will switch the measurement system to the 2, 3, or 4pole current mode, and de-activated will switch to the 4 pole voltagemode. With no mode relays activated, the system is in the 4 pole voltagemode. One relay in the input switching relays 35 always switches theanalyzer inputs to measure the voltage drop across the load resistor(s)selected in the load cell.

Programmable Load Cell

The programmable load cell 40 consists of 20 Single-Pole Single-Throw(SPST) high quality relays 41 that are connected to TP4 27 and inputmode switching relays 35. There are 20 precision load resistors 42, oneeach for the 20 relays in the cell. The resistors are connected to thenormally open terminal of the relays. These relays can be activated toswitch the resistors “in circuit” individually, or in parallelcombinations to program the required load resistance. Load cellresistance ranges can be tailored to fit specific applications byadjusting the values.

Relays for the load resistors 41 are used for load cell switching. Theload cell switching is straight forward, with no relays activated noload resistor is selected and the load value is an open circuit. Byenergizing the required relay or set of relays, one can obtain a largerange of load values.

Relay Switching and Rriver Circuitry

Relay switching and driver circuitry 45 is controlled by the TTL logicsupplied from the digital I/O card 5 resident in the computer 3. Thereare four test modes. Control channels (lines) from the I/O card controlthis mode switching. These control lines are connected to the relaydriver circuitry 46 via inverters to buffer the TTL logic to the lowimpedance of the relay drivers.

Load cell relay switching is controlled by the I/O card control lines.These control lines are connected to five 14 pin LM3146 transistorarrays in the relay driver circuitry 46. When these are activated,current is supplied to the relay coil, switching “in circuit” the loadresistor selected. The driver devices and relays can be programmed withthe computer and I/O card to select desired load values using singleresistor elements or by programming parallel resistor combinations toobtain the required load value.

The driver circuitry for the relays is configured to minimize electricalnoise by switching the current in the ground leg instead of switchingthe high side of the supply current. The load cell is laid out in a “U”configuration to keep the load resistors and control circuitry as closeto the probe jack inputs as possible. Input mode switching relays,inputs from the analyzer, and probe input jacks will employ topographyand layout considerations to minimize electrical interactions.

Power Supply

The interface board was designed to minimize power requirements from thesupply. However the power requirements exceed those of the computer andan external one is needed. The on-board power supply consists of an LM317 T, or 117 T three terminal adjustable regulator, heat sink, andassociated circuitry. The input voltage to this regulator circuit issupplied via an AC to DC converter and cable with a standard 5.0 mm plugand mating jack mounted to the interface board. This converter will havea rating between 14.5 and 16 VDC @ 800 mA to 1.0 A.

The input is filtered by two capacitors in parallel and connected to theinput terminal of the LM 317 T. The potentiometer and capacitor in thecontrol leg of the regulator sets the output voltage and reduces theripple voltage at the output. The diode and capacitor on the output legis to protect the regulator from a short circuit and to filter theoutput voltage.

b) Software

The Solder Paste and Residue Measurement System software controls thehardware used for collecting impedance data from a solder paste sample.In the preferred embodiment the software is a menu driven softwarepackage which provides the user the ability to utilize a database forsetup, and result storage, and to collect and analyze impedance data.The basic functionality of the system is illustrated in FIG. 3.

The first menu item allows the user to setup the Solder Paste in thedatabase. This information is stored in the Solder Paste Table in thedatabase. The information which may be defined within this tableincludes the Manufacturer, Model Number, Manufacturer's Specification,Circuit Description, and Experimental Notes. The Manufacturer and ModelNumber identify this record in the database. The circuit description isused to characterize the solder paste.

The second menu item allows the user to define the inspections whichwill occur on the solder paste. The information which may be enteredincludes the Solder Paste ID {Manufacturer and Model Number}, LotNumber, Item Number, Application, Viscosity Measurements, Manufacturer'sSpecifications, Impedance Data File, solder paste control (SPC) Setup,SPC Data File. Each record in the inspection table is identified by theSolder Paste ID, Lot Number, Item Number and application. The ImpedanceData File specifies the data file which will store the measuredimpedance data. The SPC Data File describes the name of the data filewhich represents the control limits for the X-Bar and R charts.

The third menu allows the user to select a solder paste inspection. Eachinspection is uniquely identified by the Manufacturer, Model Number, LotNumber, Item Number and Application. Once the user has selected a solderpaste the control parameters are loaded into the global memory so thatother features of the software may use them. The control parameterswhich are loaded include the Solder Paste ID, Inspection ID, SPC ControlLimits and Current Test Number.

The fourth menu allows the user to perform a test on the solder paste.The first action required is to collect the data from meter over avariety of frequencies. The following must be specified to collect data,Frequency Range, Probe Type, Voltage Level, Instrumentation Resistor.Once the data is collected it is characterized by curve fitting the datato the circuit description defined in the solder paste table. The solderpaste table supplies the circuit description and the starting values.The curve fit is a modified Marquardt-Levenberg non-linear least squaresfit. The resulting parameters from the curve fit are then stored in thedatabase in a record which is related to the Inspection ID. The softwarethen reviews the current and previous results and calculates the SPClimits. If a control limit is violated the software then indicates thisto the user.

The Executive Module controls the main menu, multiple documentinterface, file import/export, and printers. The Paste Database Modulecontrols access the system database. The Data Collection Module controlsthe data collection of the system. The Curve Fit Module provides thecurve fitting functionality. The RC Circuit Values Module provides SPCcontrol limit checking of X-Bar and R chart limits of specified circuitparameters.

Solder Paste and Residue Measurement System Software Flow ChartDescription

The master flow chart of the solder paste and residue measurement systemis shown in FIG. 4. At the start of the solder paste test sequence theuser must select either a 2 or 4 pole measurement scheme. 51, 52.

A flow chart of the data collection for 2-pole measurements is shown ifFIG. 5. The first step in the 2-pole measurement is setting up the GPIBcontrolled 63 within the computer. This is accomplished to initiatehandshaking protocol for future controller communication. The next stepis to setup the SR810 64. This step sends data, such as user selectedvoltage levels, low-pass filter settings, front-end couplinginformation, and grounding configuration. After initial set-up, themeter begins to step through the user selected frequencies and collectsreal and imaginary impedance using the following process:

Determine if the meter sensitivity is properly set 66.

(Meter sensitivity is a gain setting on the front end of the meter.)

If the sensitivity is set too low, the front-end amplifier willsaturate, if set too high, the measurement will not be made at anoptimum resolution.

If the meter sensitivity is too low or too high, make appropriatechanges to establish an optimum sensitivity setting.

(The sensitivity setting flow chart is shown in FIG. 8. The first stepis to set the sensitivity of the meter to the previously set sensitivityfor either the current of voltage, depending on the measurement beingmade. 112 Then determine if the meter is in overload (i.e. the front-endamplifiers are saturated) 113. If yes (meter is overloaded), then waitto allow for meter settling, and adjust sensitivity to the highestsetting and re-assess overload condition 114. If no, continue andcapture RMS voltage 115, calculate the sensitivity setting that willprovide a near 50% sensitivity 116 and reset meter sensitivity to thisnew sensitivity setting and ensure that the meter does not overload as afinal check 117.)

Continuing with in FIG. 5 with the steps required for data collectionfor 2-pole measurements the next step is to capture the voltage andphase from the series (current sensing) resistor that resides in theinterface board 68. The voltage and phase capturing logic is shown inFIG. 9. First three voltage and phase samples are read 123, 124. Thenthe mean and standard deviation of these measurements are calculated125. If the mean falls within a defined number of standard deviations,then, continue or else, Loop back to step 1, 122.

Then on the data collection for 2-pole measurements, FIG. 5 the samplevoltage drop are calculated using a voltage divider logic scheme 69, thereal and imaginary impedance of the sample are calculated given themeasured current (from the series resistor) and the calculated voltageacross the sample 70, the impedance is calculated 71, if necessary andthe results are stored in a global array (RAM) 72.

Referring back to the master flow chart FIG. 4 if the 4-pole measurementscheme is selected, then the user must select either 1 or 2 lock-inamplifiers. The preferred Solder Paste and Residue Measurement Systemdata collection in the 4-pole mode can be made with either 1 or twolock-in amps (1 measures current, the other voltage).

In FIG. 6A-B the 4-pole, 1 amp collection is shown. The first step inthe 4-pole measurement, as was with the 2-pole measurement, is settingup the GPIB controlled within the computer 76. This is accomplished toinitiate Handshaking protocol for future controller communication. Thenext step is to setup the SR810 77. This step sends data, such as userselected voltage levels, low-pass filter settings, front-end couplinginformation, and grounding configuration. After initial set-up, themeter begins to step through the user selected frequencies and collectsreal and imaginary impedance using the following process:

Switch relays in the interface board to allow for a sample voltage dropand phase measurement from the voltage sensing electrodes 79.

Determine if the meter sensitivity is properly set 80.

(Meter sensitivity is a gain setting on the front end of the meter. Ifthe sensitivity is set too low, the front-end amplifier will saturate,if set too high, the measurement will not be made at an optimumresolution. If the meter sensitivity is too low or too high, makeappropriate changes to establish an optimum sensitivity setting. Referto FIG. 8 and discussion supra for more details regarding sensitivitysetting logic 81.)

Capture the voltage and phase from the voltage sensing electrodes 82(for voltage and phase capturing logic, refer to FIG. 9 and discussionsupra).

Switch relays in the interface board to allow for a sample current andassociated phase measurement 83.

Determine if the meter sensitivity is properly set 84.

(Meter sensitivity is a gain setting on the front end of the meter. Ifthe sensitivity is set too low, the front-end amplifier will saturate,if set too high, the measurement will not be made at an optimumresolution. If the meter sensitivity is too low or too high, makeappropriate changes to establish an optimum sensitivity setting. Referto FIG. 8 and discussion supra for more details regarding sensitivitysetting logic.)

Capture the voltage and phase from the series 86 (current sensing)resistor that resides in the interface board (FIG. 9). This is themagnitude and phase of the current flowing through the sample.

Calculate the real and imaginary impedance 87 using the sample voltageand current measurements as described above.

Calibrate the impedance 88, if necessary.

Store data in a global array 89 (RAM).

For 4-pole, 2 amp collection, refer to FIGS. 7A-B The first step in the4-pole measurement, as was with the 2-pole measurement, is setting upthe GPIB controlled within the computer 93. This is accomplished toinitiate Handshaking protocol for future controller communication. Thenext step is to setup the SR810 94. This step sends data, such as userselected voltage levels, low-pass filter settings, front-end couplinginformation, and grounding configuration. After initial set-up, themeter begins to step through the user selected frequencies and collectsreal and imaginary impedance using the following process:

Switch relays in the interface board measure voltage and phase data fromLock-In Amp #1 96.

Determine if the meter sensitivity is properly set 98. (See FIG. 8 anddiscussion supra.)

Capture the voltage and phase from the voltage sensing 100. (See FIG. 9and discussion supra.)

Switch relays in the interface board measure voltage and phase data fromLock-In Amp #2 101.

Switch relays in the interface board to allow for a sample current andassociated phase measurement 102.

And once again determine if the meter sensitivity is properly set. (FIG.8 and discussion supra.)

Capture the voltage and phase from the series (current sensing) resistorthat resides in the interface board 105. This is the magnitude and phaseof the current flowing through the sample.

Calculate the real and imaginary impedance using the sample voltage andcurrent measurements as described supra 106.

Calibrate the impedance 107, if necessary.

Store data in a global array (RAM) 108.

Back to FIG. 4, that master flow chart, once the data is collected, itis analyzed using Complex Nonlinear Least Squares (CNLS) Curve FittingAnalysis. The logic of this portion of the software is shown in FIG. 10.After the collected data is retrieve the following data from the database 130 the circuit structure is determined (i.e. series & parallelconstruction of the resistors and capacitors) and the starting valuesfrom which the iterative curve fitting process will commence aredetermined. A mathematical function is constructed 131 from the circuitstructure provided that calculates the real and imaginary impedance. Theimpedance data from the solder paste test is loaded into arrays 132 andusing the constructed mathematical function and the collected data,enter the curve fit minimization algorithm (Marquardt-Levenberg Method)134. This minimization method is described in FIGS. 11A-B In the firstiteration, the variable alambda is set to 0.00001 143 and the error(chi-square) is calculated 144 from the starting values provided fromthe data base. If this is not the initial iteration continue to 145. Acovariance matrix is created 145 and a linear solution to the covariancematrix (Gauss-Jordan is used) is found 146. The next guess for theparameters (This is accomplished using the solution to the covariancematrix and the alambda value.) is then calculated 147 and the error(chi-square) using the new set of estimated parameters is calculated148. At this point, at 149, the new set of estimated parameters aretested to determine if the error has decreased. If the new guess lowersthe error alambda is adjusted down 151, if the new guess does not lowerthe error alambda is adjusted up 150 and the chi-square (error) is set152 to the chi-square of the previous iteration. Finally, the selectedparameters are adjusted with current guess and the process continues.From this curve fit minimization algorithm, error (chi-square) andestimated component parameters are provided.

Continuing with FIG. 10, the non-linear least squares curve fit, afterthe curve fit algorithm described supra the program logic enters thedecision block 135 and the error from the Marquardt-Levenberg isdetermined to be acceptable or unacceptable. If the error is acceptablethe error is calculated from the Covariance Matrix, store estimatedparameter values, and count as an acceptable interation 137. The programthan loops back to 133 until the number of specified iterations are metwith no change in error and the iterative changes for each parameter areoscillating. If the error from the non-linear least squares curve fitalgorithm is not acceptable then determine if the error is worse thanthe previous iteration? If it is not then clear the count of acceptableiterations and go to 133, otherwise go directly to 133

After the data is analyzed using the CNLS analysis techniques, best-fitcircuit parameter are generated that are compared with the user definedStatistical Process Control (SPC) limits. FIGS. 12A-B shows the flowchart for testing the SPC limits. The first step in this testing is toextract the current SPC limits from the data base 156. Then for each ofthe parameters selected for a specific component configuration for aspecific solder paste do the following, (Note: this is used to developthe SPC charts—X—Bar and R Charts):

For every test in the inspection set for a particular solder paste 157A,retrieve the estimated value for the parameter 159A. Calculate the sum,the sum of squares, and the square of the estimated parameter value forthe inspection set 160 A. Calculate the Sample Mean and the StandardDeviation 162 and determine if the control limits fixed by the user havebeen surpassed 163? If they have continue to 165. If they have not thencalculate the control limits based on the sample mean, standarddeviation, and the user selected K value (K values are used in SPCsystems to expand and contract the control of a process) 164. Finallythe parameter collected from a specific test is tested to determine ifit violates the control limits and if it has the violation is reported166 otherwise the program loops back to 157 till all parameters havebeen tested.

In the final stage in the master flow chart, FIG. 4, the results of thetest are displayed and stored in the data base 60 and the program andimpedance spectroscopy measurement has completed.

c) Manufacturing Interface & Modeling

The utility of using impedance spectroscopy on solder paste and residuematerials is to provide more control in their use in manufacturing.Solder paste specifically is a very dynamic material that can readilydeteriorate within a single shift in manufacturing. A generic list ofsolder paste failure modes is as follows:

a) Moisture absorption causing excessive powder oxidation and solderballing

b) Changes in rheologic properties (Viscosity and Thixotropic Index)

c) Increased powder oxidation state

d) Inactivation or immobility of the activator

Therefore, in order for this system to be implemented as an solderprocess control (SPC) device for solder paste, there must be strongcorrelations between the data generated by this measurements system andthe behavior of solder paste in manufacturing. In order to provide thislevel of correlation both linear regression techniques and probabilisticfailure analysis techniques were employed.

The approach of Impedance Spectroscopy is to electrically map physicalchanges in materials using electrical equivalent components. The logicassociated with implementing impedance spectroscopy (IS) for solderpaste materials is provided in FIG. 13. The first step in implementingIS techniques for controlling solder paste is to design a solderpaste/residue probing system. The probing system implemented has a greateffect on the physical phenomenon measured with the solder paste. As anexample, a four probe measurement scheme concentrates on measuring thebulk behavior of the material, a 3 probe system measures interfacebehavior (I.E., reaction, diffusion, adsorption, etc.), and a 2 probemeasurements incorporate both interface and bulk behavior. The next stepis to design an experiment that will cause changes in the solder pastematerial that resembles the type of changes that can be seen inmanufacturing. After the experiment is designed, the next step is toestablish theories and models that map the behavior of the solder pasteand to develop an equivalent electrical circuit that maps the samebehavior. Once the appropriate equivalent electrical circuit is found,curve fitting routines are employed to derived specific component valueswithin the electrical circuit. From this point on, the characteristicchanges in the solder paste can be correlated with specific componentsor sets of components within the equivalent circuit.

Moisture Absorption and Its Effect on Manufacturing Yields

One of the manufacturing anomalies associated with solder paste ismoisture absorption from the surrounding environment. Absorbed moisturecan cause two changes within the solder paste 1) excessive powderoxidation due to the moisture acting as a catalyst for acceleratedoxidation and 2) solder balling due to volatilization of the water vaporduring the reflow process. A number of publications by MarquetteUniversity (M. Polcyznski, M. A. Seitz, and R. Hirthe, A New Techniquefor Monitoring Solder Paste Characteristics Proc. of the 14th AnnualElectronics Manufacturing Seminar, Naval Weapons Center, China Lake,Calif. (1990). M. Polcyznski, M. A. Seitz, and R. Hirthe, A NewTechnique for Monitoring Solder Paste Characteristics Surface MountTech. 4, p. 54-60, (1990), M. Polcyznski, M. A. Seitz, and R. Hirthe,Measuring Solder Paste Metal Content Using Altemating Current ElectricalImpedance Techniques Proc. 1990 International Symposium onMicroelectronics, Chicago, Ill., Oct. 15-17, p. 174-182 (1990), M. A.Seitz, and R. Hirthe, Thermal Stability of Metal Oxide Surge SuppressionDevices, 1990 EOS/EDS Proceeding, Lake Buena Vista, Fla., Sept. 11-13,P. 187-192, (1990), M. Polcyznski, M. A. Seitz, and R. Hirthe,Microstructural Mechanisms Associated with the Electrical ImpedanceCharacteristics of Solder Paste Flux, Proc. of the 1 5th AnnualElectronic Manufacturing Seminar, Naval Weapons Center, China Lake,Calif. (1991), M. Polcyznski, M. A. Seitz, and R. Hirthe, Use of ACElectrical Impedance Techniques for Monitoring Microstructural Changesin Electronic Materials, Proc. of the 1991 International Sym. onMicroelectronics, Orlando, Fla., Oct. 21-23, P. 431-435, (1991), M. A.Seitz, and R. W. Hirthe, M. Amin, and M. Polcyznski, Monitoring SolderPaste Properties Using Impedance Spectroscopy, Proc. of the 1992International Symposium on Microelectronics, San Francisco, Calif., Oct.19-21, P. 503-509, (1992), M. A. Seitz, and R. Hirthe, M. Amin, and M.Polcyznski, Low Frequency Electrical Behavior of Solder Paste, Proc. ofthe 16th Annual Electronics Manufacturing Seminar, Naval Weapons Center,China Lake, Calif. (1993), M. A. Seitz, and R. W. Hirthe, M. Amin, ACElectrical Characterization of Solder Paste, Proc. Electrecon 93,Indianapolis, Ind., May 19-21, P. 14.1-14.18, (1993) ) have shown thatthe impedance data for solder paste changes based on different exposuretimes and amount of moisture in air. The present invention was able tolink this IS data to solder ball failures in solder paste. In order tosubstantiate this position, a 2-probe IS experiment was conducted withan Alpha RMA 390 and different humid environments and exposure times.The electrical circuit that was used to model the RMA solder paste in a2 probe configuration was a simple resistor and capacitor in paralleland the frequency range chosen, among many possible range choices, ofthe stimulus AC waves were from 5 Hz. to 10 kHz. The resistor, in thiscase, tracked well with the probability of obtaining solder balls inmanufacturing. The results of the experiment can be found in FIG. 14.Where the vertical line through the middle of the graph is the lowercontrol limit for resistance and the horizontal line through the graphshows where less than one out of three solder paste patterns willcluster and thus above this horizontal line is unacceptable solderballing and below this line is acceptable solder balling. The y-axis isthe level of solder balling (i.e. the severity of solder balling on anumeric scale) and the x-axis is the value of the parallel resistor.FIGS. 15A-D shows a graphic representation of the numeric scale forsolder balling. FIGS. 15A-D show solder balling characteristics on ascale from 1 to 4 with 1 being preferred, (FIG. 15A) 2 being acceptable(FIG. 15B), 3 being unacceptable (clusters) (FIG. 15C) and 4 beingunacceptable (FIG. 15D).

Data from the manufacturing floor, in an optimum environment showed thatthe resistor value stayed between 0.8×10⁷ and 1×10⁷; therefore it can beseen that the lower control limit established on R for this solder pastewould be between 5-6×10⁷ in order to avoid solder balling. Moistureabsorption can also be mapped using 4 probe instrumentation. The resultsof another short experiment on the same solder paste is shown in FIG.16A. Some interesting differences between the 2 probe and the 4 probedata is that there was an evolution of another low frequency RC parallelbehavior in the 4 probe configuration. The 2 probe data was measuringthe high impedance of the flux-probe interface while the 4 probetechnique was measuring the impedance of the flux powder interface.FIGS. 16B-C show how the 2 time constants behaved over time and how itcorrelated with printability characterists (i.e., skips). Upper andlower control limits are then generated to provide control for use ofthe solder paste. The upper control limit is at approximately 7.25 E-04and the lower control limit is at approximately 6.20 E-04 with slipsbeginning to occur at approximately at 10. In FIG. 16C the upper controllimit is the top dashed line in the graph and the lower control limit isthe lower dashed line in the graph and the point where slips begin tooccur is at 13.

Changes in Rheologic Properties

Another property of solder paste that is of interest is the rheologicnature of the solder paste over time. Any changes in the viscosity orthixotropy behavior can have disastrous effects on the ability to printthe paste on a circuit card. There are two properties that aretraditionally tracked with solder paste: 1) viscosity and 2) thixotropicindex. In experiments the rheologic properties were tracked with the ISelectrical models. The first paste that was experimented with was an AIMWater Soluble 437 solder paste. The inventors of the present inventionwere able to map the viscosity change with some of the bulk timeconstants within the circuit model. The circuit model used for thispaste is described in FIG. 17. The circuit model was developed over afrequency range from 5 Hz. to 13 Mhz. ( the measurements were made byintegrating a higher frequency impedance measuring device.) The abilityto track rheologic properties of the solder paste is heavily dependenton the chemistry of the solder paste. With the AIM WS 437 we had adirect linear relationship between the highest frequency time constantand viscosity. Using linear regression techniques, we had a R2 valuegreater than 0.95! See Table 2 for the fit characteristics for each ofthe three time constants and viscosity at different shearing rates.

High Freq Mid Freq Low Freq Interface Tau-1 Tau-2 Tau-3 Tau-4 ImmediateMeasurement Rsq(10) = 9.92E−01 7.11E−01 3.88E−01 4.57E−03 Slope(10) =1.03E+10 4.60E+07 1.27E+05 2.43E+05 Rsq(4) = 9.75E−01 6.75E−01 6.08E−011.87E−02 Slope(4) = 1.51E+10 6.63E+07 2.35E+05 −7.28E+05  Rsq(0) =9.43E−01 6.44E−01 6.83E−01 4.39E−02 Slope(0) = 1.83E+10 7.98E+073.07E+05 −1.38E+06  Rsq(TR) = 7.00E−01 4.48E−01 9.10E−01 2.35E−01Slope(TR) = −8.01E+07  −3.38E+05  −1.80E+03  1.62E+04 Rsq(10) = 9.17E−012.62E−01 9.62E−01 8.98E−01 Slope(10) = 6.08E+09 1.27E+07 5.08E+052.09E+06 Rsq(4) = 8.71E−01 3.59E−01 9.49E−01 9.71E−01 Slope(4) =1.03E+10 2.59E+07 8.82E+05 3.80E+06 Rsq(0) = 8.43E−01 3.85E−01 9.29E−019.76E−01 Slope(0) = 1.32E+10 3.47E+07 1.13E+06 4.94E+06 Rsq(TR) =7.13E−01 4.51E−01 8.19E−01 9.43E−01 Slope(TR) = −7.11E+07  −2.20E+05 −6.22E+03  −2.84E+04  After 5 Minutes

Table 2 R squared values and Slope Values Associated with IS TimeConstant Correlations with Solder Paste Viscosity on AIM 437 SolderPaste

Experiments were also conducted on a Multicore Water Soluble SolderPaste LG02. One of the characteristics of the LG02 was a presence of afluorinated activator. The electrical circuit that tracked with the LG02was a somewhat different from the AIM water soluble in that there was aactivator diffusion rate that was significantly from the activator—oxidereaction rate. A graphical representation of the LG02 equivalentelectrical circuit can be found in FIG. 18. This circuit establishedusing the same frequency as the AIM 437 described supra. One of thecharacteristics of the LG02 was the conversion of the S_(π)—Oxide to anO—S_(π)—F compound. This compound formation had a drastic effect on thethixotropic nature of the solder paste.

The inventors believe that the flouridic conversion of the S_(π)O oxidecaused the solder paste material to shear thin more readily under highstress. This shear thinning phenomenon was tracked by an increase in thethixotropic index. In FIG. 19, Tau-2 was tracking this flouridicconversion. Once the O_(π)—S_(π)—F formed on the surface of the powder,the thixotropic behavior of the solder paste went through a radicalchange. It is a well understood phenomenon that the state of the powdersurface has a dramatic effect on the rheology character of the solderpaste. Using FIG. 21, we would establish a upper control point for tau-2at around 1×10−4 to ensure a consistent thixotropic behavior inmanufacturing.

Increased Powder Oxidation

Experiments were also performed on solder powder to determine theability of IS measurement techniques to track different characteristicsof powder oxidation. Five different powders that have been aged at roomtemperature conditions from 1 to 5 years were used. The oldest powderwas manufactured in 1989 and newest was 1994. The powder was placed in a0% activator Multicore RMA flux paste and then modeled using theequivalent circuit described in FIG. 20. The Correlation between the ageof the powder (i.e. the amount of oxidation) and the IS data is providedin Table 3. This shows that the 4-probe techniques are sensitive todifferent amounts of powder oxidation.

TABLE 3 Relationship between Powder Oxidation and IS data. Date ofPowder Particle Size Tau-2 1989 45 m 1.7 × 10−3 1990 75 m 1.2 × 10−31990 53 m 8.2 × 10−4 1991 45 m 7.8 × 10−4 1994 45 m 6.9 × 40−4

d) Probing Hardware

The probes that have been designed to measure solder pastes and residuesexist as either of a bulk or surface type. The bulk probes are designedto minimize the environment as a factor when making the IS measurementswhile the surface probes allow the environment to interact with thesolder paste.

FIG. 21 illustrates the surface 4 probe of the present invention. Thesurface probe seen in FIG. 21 is designed to have large symmetriccurrent plates and small voltage probes. The small voltage probes areused to avoid disturbing the electric field. The spacing between thevoltage probes is also very important in that, being to narrowly spacedmay cause intermittent shorting between the probes and to much spacebetween the probes will not allow for an accurate measurement. A spacingof 0.040″ was found to work for solder paste being tested. This surfaceprobe was designed to be placed under a screen printer. Spacings lessthan 0.020″ experienced intermittent shorting problems. In FIG. 21 4surface probes are shown, probe has two large rectangular areas, 200,201 that are the current plates of the probe, and two small innercircular areas 202, 203 that form the voltage probes of the probe. Thecurrent plates can vary between a width of between 0.10 of an inch to0.50 of an inch and the length can be just about any reasonable lengthgreater than 0.375 of an inch. The diameter of the circular areas canvary from 0.02 to 0.06 of an inch with the optimum being 0.035. Thespacing between each voltage probe 202, 203 and the current probes 200,201 must be greater than 0.02 of an inch and the spacing between eachvoltage probe must also be greater than 0.02 of an inch.

FIGS. 22 and 23 show the bulk ¾-pole bulk probe. This bulk is designedto minimize the environment as a factor when making the IS measurements.As is the case with the surface probe, the spacing of the electrodes isimportant for the bulk probes. (See FIGS. 24 and 25 for a completelisting of spacing.)

In the three probe measurement technique, the measured current is thatwhich flows from the working electrode to the counter electrode, whilethe potential is measured between the reference electrode and thecounter electrode.

In the four probe measurement technique, the outer current electrodesare separated from the voltage measuring electrodes. Impedances arecalculated using the current flowing through the specimen, and thevoltage across the set of inner voltage probes. Since the voltagemeasuring electrodes do not draw current and are placed inside thecurrent supplying electrodes, electrode impedances are excluded from theimpedance measurement. Thus, the electrical behavior of the bulk of thespecimen can be extracted from samples that have significant electrodeimpedances.

What is claimed is:
 1. A 4 probe impedance solder paste probe formeasuring surface samples of solder paste, comprising: a printed circuitboard with a dielectric constant of a K value between 2.0 and 7.0; tworectangular current plates etched on said printed circuit board andplaced opposite to each other; and two voltage probes etched on saidprinted circuit board, placed opposite each other on the board andplaced on said board between said current plates.
 2. A 4 probe impedancesolder paste probe for measuring surface samples of solder paste asrecited in claim 1 wherein said current plates have a width of between0.10 to 0.50 of an inch and a length greater than 0.375 of an inch, andwherein said voltage probes are circular and have a diameter of between0.02 and 0.06 of an inch.